This paper presents a longitudinal study of an elementary mathematics teacher and describes her beliefs system in terms of Green's (1971) metaphor and provides examples of how her beliefs were enacted in her classroom practices. Dewey's (1933) notion of reflective thinking is used to explain the changes in the structure of the teacher's belief system. The purpose of the paper is to illuminate aspects of the structure of the teacher's belief system that enabled her to change some of her beliefs in a surprisingly short period of time. (Contains 10 references.) (Author/MM)